1. Field of the Invention
The present invention relates generally to multilevel lithographic contact masks and to a method for producing such masks. The tool is useful in the preparation of LIGA plating molds made from PMMA, or similar materials. In particular the tool is useful for providing an ability to apply a graded, variable exposure dose across a photosensitive substrate. By controlling the exposure dose from point-to-point, it is possible to control the development process for removing exposed portions of the substrate; adjusting it so as to proceed more or less uniformly regardless of feature size or feature density.
2. Background
Preparing microparts by the well known xe2x80x9cLIGAxe2x80x9d process (LIGA is an acronym based on the first letters for the German words for lithography and electroplating. A general review of the LIGA process is given in the article by W. Ehrfeld, et al., xe2x80x9cLIGA Process: Sensor Construction Techniques Via X-Ray Lithography,xe2x80x9d Technical Digest IEEE Solid State Sensor and Actuator Workshop, 1988, pp. 14-4.) requires producing intricate molds onto which the desired parts may be built-up, typically by plating or a similar deposition process. A variety of methods are known for making microparts such as are described in the patent literature. In particular, U.S. Pat. Ser. Nos. 5,162,078, 5,378,583, 5,527,646, 5,631,514, 5,679,502, and 5,917,260, describe methods and techniques for forming microparts using x-ray radiation as a light source for creating molds for said parts. Most current techniques rely in some manner on using a photosensitive plastic mold fabricated from a material such a PMMA. These molds are typically patterned by illuminating the substrate with a high energy synchrotron beam partially blocked by a radiation-opaque mask. Those areas of the substrate which are lit by the beam experience radiation damage which xe2x80x9ccutsxe2x80x9d the polymer chains comprising the substance of the substrate to some effected depth. This damaged material is rendered vulnerable to chemical attack and can therefore be xe2x80x9cdevelopedxe2x80x9d or dissolved away to provide an etched image in the substrate material. However, as more intricate molds are prepared, wherein ever smaller and more closely packed mold features are incorporated into the replicated pattern, mass transport limitations considerably slow the development process in areas which are restricted by lateral size and/or depth. These confined spaces make it difficult for fresh developer to get into and out of these narrow areas resulting in a potential under-development for a given processing time. This limitation begins to create severe problems when developing mold patterns which include features having widely varying packing densities.
In particular, it is known that chemical dissolution kinetics of PMMA are greatly enhanced by the damage induced in those areas exposed to a flux of x-ray radiation thus making it possible to utilize PMMA as a lithographic medium. It is also known that given sufficient time the developer will attack even the unexposed portions of the PMMA. Suppose now that a PMMA mold is prepared to provide a micro-gear that includes a central post to form a central mounting hole in the micro-gear. Such an example is shown, in cross-section, in FIG. 18A Furthermore, suppose that in the same mold a narrow slot is provided for the fabrication of a pusher rod or arm. In this example, material is rapidly removed in the volume of the substrate corresponding to the micro-gear body due to its large relative size, while the material of the adjacent narrow slot reacts much more slowly. However, as shown in FIG. 18B, if one were to wait until the narrow slot is fully developed, the unexposed portion (post) at the center of the micro-gear mold depression, corresponding to the shaft of the micro-gear, is observed to become severely undercut due to its prolonged exposure to the developer solution. Furthermore, if one waits only until the volume of material around the post at the center of the mold is removed, a substantial volume of the exposed PMMA body comprising the narrow slot remains undeveloped (FIG. 18C).
However, by adjusting the radiation dose in various regions of the mold, such that larger, low aspect regions are underexposed while the narrow, high aspect regions are fully exposed, FIG. 19A, it should be possible to develop both regions simultaneously as shown in FIG. 19B.
In general, therefore, in cases where features are locally isolated, i.e., features which have relatively large volumes of material to be removed around them, the developer reaction acts quickly due to the ability of the developer solution to easily migrate to the reacting surfaces of these areas of the mold. However, those areas having numerous closely packed features, or which exhibit a few features which are relatively small and surrounded by narrow channels, the developer reaction proceeds much more slowly. This difference in the rates of development, therefore, presents a technical challenge needing to be overcome in order to produce molds for making a wide range of micropart-s having widely varying feature sizes.
The prior art does not discuss the problem of simultaneously developing an exposed photosensitive substrate having both closely packed small features and widely separated larger features, nor does it address the advantage of a system for applying a stepped or graded exposure dose to the substrate in order to control the degree of exposure dosing and thus the rate at which different areas of a substrate developed. What is needed, therefore, is a method, and a tool for practicing the method for simultaneously providing different radiation dosing to a plurality of regions on the substrate mold. What is needed is a mul-tizone mask wherein each zone is tailored to pass a specific quantity (dose) of radiation and thus allow for developing a mold at a uniform rate regardless of feature location, configuration, or packing density.
It is, therefore, an object of the present invention to provide a lithographic mask tool for providing a variable range of radiation dosing across the entire useful surface of the mask.
It is another object to provide an x-ray mask comprising a silicon substrate having a metal pattern deposited into one or both surfaces of the substrate wherein the metal pattern is present in one or more thickness.
Yet another object of the invention is to provide a mask having an image forming pattern, wherein the pattern exhibits a range of thicknesses providing varying degrees of radiation attenuation from point to point across the mask surface.
It is another object of the invention to provide an x-ray mask wherein the metal pattern comprises a plurality of structural elements on both faces of the mask substrate.
Still another object of the invention is to provide an x-ray mask wherein the structural elements comprising the pattern cover varying portions of the substrate surface and are also embedded into trenches etched into the substrate surface.
Still another object of the invention is to provide an x-ray mask comprising two or more overlaying layers of an x-ray attenuating material on one surface of a silicon substrate.
These and other objects will become clear to those having skill in these arts as the invention is described more fully below.